Solid polymer electrolyte (SPEs) containing dissolved metal salts have been proposed as an alternative to liquid electrolytes in rechargeable electrochemical devices. There are many advantages to using a solid electrolyte, such as the capability for high speed production of thin cells constructed in a bipolar arrangement. Further, the electrolyte can act as a mechanical separator between the anode and cathode, eliminating the need for an inert porous separator, as well as acting as a binder/adhesive to move and conform to electrode volume changes during cycling. Because the system would be all solid state, cells of any desired geometric shape would be possible. There is also a safety advantage in that if the integrity of the sealed cell package is broken, there is no liquid to leak out.
One polymer system of interest is based on high molecular weight poly(ethylene oxide), HO--CH.sub.2 CH.sub.2 (O--CH.sub.2 CH.sub.2).sub.n OH. An ionically solid polymer electrolyte can be prepared by dissolving PEO and an appropriate salt in a suitable volatile solvent such as acetonitrile. By solution casting, acetonitrile is removed by evaporation, leaving a free standing, solid, flexible film of good mechanical strength that contains only PEO with dissolved salt. Such films are ionic conductors. Unfortunately, at room temperature, these PEO films are highly crystalline so ionic conductivity is poor (.apprxeq.10.sup.-7 Scm.sup.-2) rendering PEO as an impractical electrolyte at these temperatures.